The isolation device must also, in certain cases, fulfill another function, at first sight in conflict with that presented hereinabove, which is to attenuate dynamic loads, such as vibrations and shocks, large and transient, applied to the supporting structure and propagating through the isolation device as far as the equipment, which could be sensitive to these loads.
In order to facilitate the understanding of the invention, the latter is described hereinafter more particularly in the framework of certain applications associated with the space industry, and for which the invention presents a particular interest for the applicant.
It is well known that some actuators, commonly used for the attitude control on satellites, contain one or more rotating elements, whose balancing, which is always imperfect in practice, generates vibrations propagating through the structure of the satellite. This is the case, for example, for reaction wheels, momentum wheels, control moment gyroscopes (or CMG), flywheels, etc.
Other equipment installed on board satellites may also generate such vibrations, such as, for example, refrigerators using compressors.
The vibrations created by this equipment may be transmitted as far as payloads of the satellites and degrade their performance, as is the case, for example, for optical observation instruments that are particularly sensitive to the movements of the sightline that are detrimental to the quality of the images obtained.
In order to counter these damaging effects, according to the prior art, the interfering equipment or an interfering assembly of equipment is mounted on an isolation device forming a mounting interface for the equipment or assembly of equipment on the supporting structure. In addition to this mounting function, the isolation device must attenuate the transmission of the vibrations generated by at least one piece of equipment according to a certain frequency bandwidth depending on the application. In the case where at least one piece of equipment is an actuator in torque or in force, the isolation device must also transmit the useful torques or forces generated by this equipment according to a transmissibility frequency bandwidth also specified by the user. On the other hand, it is also desirable, if not necessary, that the isolation device attenuate the very high dynamic loads applied to the supporting structure during the launch phase of the satellite, which seems to be contradictory with the function for isolation of the vibrations of at least one piece of equipment, since these vibrations are several orders of magnitude smaller than the dynamic loads at launch.
The isolation device according to the invention must therefore operate:                on the ground, in particular during the performance tests and the qualification tests carried out prior to the launch of the satellite,        during the launch of the satellite, in particularly challenging environments with vibrations, acoustic noise and shocks, and        when the satellite is in orbit, where the performance for isolation from very small vibrations will need to attain the expected levels.        
Lastly, the isolation device according to the invention must also sometimes maintain over time a certain alignment stability of the vibrating equipment with respect to the supporting structure.
The following numerical values correspond to a typical operating range of one particular embodiment of the device of the invention when it is applied to the isolation of actuators of the reaction wheel or CMG type on board satellites:                mass of a vibrating piece of equipment to be isolated: typically from 1 kg to 30 kg;        interfering forces and torques generated by the vibrating equipment: 1 N to 100 N over a frequency range from 10 Hz to 1000 Hz;        desired attenuation factor for vibrations and shocks: 3 to 50 in the range 10 Hz to 1000 Hz (typically an attenuation with a slope of −2 in log scale over the frequency range);        overload factor of the isolation device: less than 2;        desired transmissibility factor: 1±5% in the range 0 to 10 Hz;        alignment stability over time: able to go typically up to 0.05°;        environment at launch: typically 20-100 g in the range 10-100 Hz (vibrations) and 1000 g in the range 100-1000 Hz (shocks) (g being the acceleration of the earth's gravitational field).        
At this point in the description, it should be noted that the device provided by the invention which is presented hereinbelow may equally be used to isolate, for example an assembly of equipment, rather than a single piece of equipment, some of which cause interference since they are vibrating, and all mounted onto the same equipment-holder platen. The isolation device according to the invention may also be used at the interface between one part of a satellite, for example a service module, and another part of the satellite, for example a payload. In these two cases, the numerical values hereinabove, given by way of example, are not necessarily applicable.
From the U.S. Pat. No. 5,305,981 and U.S. Pat. No. 5,971,375, multi-axis isolation devices are known that comprise a plurality of isolation studs disposed between two structural parts, one of which carries vibrating equipment, and the other is to be fixed to the supporting structure of the satellite. For example, U.S. Pat. No. 5,305,981 describes a device composed of 6 isolation elements in a symmetrical arrangement in a symmetrical hexapod configuration of 3 pairs of oblique studs providing a viscous damping and an attenuation of the vibrations and shocks when the satellite is launched and in operation in space. In this device, the isolation elements are mounted onto links with two degrees of freedom allowing axial play and the flexing moments to be minimized. Buffers situated between the two structural components of the device limit the excursions of the isolation elements, in particular when the satellite is launched. It should be noted that with this concept, 6 isolation elements are required in order to achieve total isolation from forces and torques along the three axes. The mounting of the device onto the structure of the satellite requires an intermediate structural part. The buffers are not integrated into the isolation elements. The design is not modular. A generalization of the design to other types of isolation is not straightforward, in particular, when the number of degrees of freedom to be isolated varies. The adjustment of the device as a function of the specifications for isolation from the small vibrations generated by the equipment and for isolation from the high levels of vibrations and shocks during launch, together with the transmissibility of the useful torques generated by the equipment, is not straightforward. The size of the device, its mass, its lack of modularity are not favorable to its design, its construction or its application.
The device described in U.S. Pat. No. 5,971,375 has a similar configuration. The same limitations are apparent, with in addition a complex layout of a plurality of springs.
From the patent document JP 2000 145889, inserts for mounting a piece of equipment onto a supporting structure are also known that comprise (see FIGS. 12 and 13) two rigid components, one external 112, the other internal 115, rigidly attached to the equipment and connected to one another by means of a plurality of isolation studs 118 made from elastomer, and such that the internal component 115 comes to a stop against the external component 112 whenever the load applied to one of the studs 118 exceeds a permitted limit. In the case considered here of an equipment isolation system on board satellites having to withstand the launch, the application of the principle described in JP 2000 145889 necessarily leads to the installation of a large number of buffers 119 between said internal components 115 and external components 112, in order to block all of the degrees of freedom in translation and tilting of the internal component 115 with respect to the external component 112, owing to extremely high loads to which the device is subjected along these degrees of freedom during launch. One embodiment of such a device is presented in FIG. 12 corresponding to the device of the document JP 2000 145889. This type of insert, whose necessary complexity of design and construction is highlighted, could possibly be used as such for isolating the small vibrations generated by a piece of equipment. However, its design is such that, when the equipment and its supporting structure are subjected with respect to one another to large movements and accelerations with vibrations and shocks, for example during the launch phase of the satellite, several, rather than only one, buffers 119 of the insert will come into action simultaneously, for example as is shown in FIG. 13 during tilting, due to moments M. Since all the moments can generate excessive stresses on the studs 118, they must be blocked by buffers 119. This will create enormous local forces FL within the insert, and will inevitably lead to its destruction, since, as previously mentioned, the high-frequency vibrations sustained during the launch are of a very high level. In order to avoid this, it will be necessary to block all the relative movements of the internal component 115 and external component 112 by a rigid connection, by means of an appended stacking device. It will subsequently be necessary to unlock this stacking device once in orbit, by means of pyrotechnical or thermal elements, which will render the overall isolation system substantially more complex.
The problem on which the invention is based is to provide a multi-axis isolation device for at least one piece of equipment that generates vibrations, installed on board a supporting structure such as a satellite, and of the type presented hereinabove, and which overcomes the aforementioned drawbacks of the prior art and is more suited to the various demands encountered in practice than the known devices, in particular according to the aforementioned US and JP patent documents.